internal/subtle: document and test XORBytes overlap rules #272

internal/subtle/xor.go

@@ -4,10 +4,15 @@
 
 package subtle
 
+import "github.com/emmansun/gmsm/internal/alias"
+
 // XORBytes sets dst[i] = x[i] ^ y[i] for all i < n = min(len(x), len(y)),
 // returning n, the number of bytes written to dst.
 // If dst does not have length at least n,
 // XORBytes panics without writing anything to dst.
+//
+// dst and x or y may overlap exactly or not at all,
+// otherwise XORBytes may panic.
 func XORBytes(dst, x, y []byte) int {
 	n := len(x)
 	if len(y) < n {
@@ -19,6 +24,9 @@ func XORBytes(dst, x, y []byte) int {
 	if n > len(dst) {
 		panic("subtle.XORBytes: dst too short")
 	}
+	if alias.InexactOverlap(dst[:n], x[:n]) || alias.InexactOverlap(dst[:n], y[:n]) {
+		panic("subtle.XORBytes: invalid overlap")
+	}
 	xorBytes(&dst[0], &x[0], &y[0], n) // arch-specific
 	return n
 }

internal/subtle/xor_test.go

@@ -58,6 +58,14 @@ func TestXorBytesPanic(t *testing.T) {
 	mustPanic(t, "subtle.XORBytes: dst too short", func() {
 		subtle.XORBytes(make([]byte, 1), make([]byte, 2), make([]byte, 3))
 	})
+	mustPanic(t, "subtle.XORBytes: invalid overlap", func() {
+		x := make([]byte, 3)
+		subtle.XORBytes(x, x[1:], make([]byte, 2))
+	})
+	mustPanic(t, "subtle.XORBytes: invalid overlap", func() {
+		x := make([]byte, 3)
+		subtle.XORBytes(x, make([]byte, 2), x[1:])
+	})
 }
 
 func BenchmarkXORBytes(b *testing.B) {